PROJECT SUMMARY/ABSTRACT While an infectious etiology has been hypothesized in systemic lupus erythematosus (SLE) for more than a century, all studies have failed to identify a microbial cause of this autoimmune disease. This has led to the search for endogenous drivers of sterile inflammation and type I interferon (IFN-I) production in SLE. Over 40 percent of the human genome is made up of hundreds of thousands of repetitive sequences that evolved from genetic elements called transposons, which contain virus-like sequences. Retrotransposons are the predominant class of transposable elements in most mammalian genomes. In recent years, it has been found that similar to viral products, nucleic acids derived from these genomic ?parasites? can activate cytosolic nucleic acid sensors. These findings have linked dysregulated expression of retrotransposons with diseases characterized by sustained IFN-I production, such as SLE. In particular, defective regulation of a subclass of non-LTR retrotransposon, the long interspersed nuclear element-1 (LINE1 or L1), has been associated to SLE pathogenesis. To gain further insights into the potential role of L1 elements in SLE, we focused on neutrophils and IFN-I, two important players in SLE pathogenesis. L1 contains two open-reading frames (ORF1 and ORF2). Using SLE neutrophils with evidence of IFN-I activation, we initially identified a novel polymorphic variant of ORF1. Using the protein (ORF1p) encoded by this variant as an antigen in preliminary studies, we identified for the first time that patients with SLE have antibodies to L1-ORF1p. This supports the notion that L1 elements are active and their products stimulate the immune response in SLE. In addition to these innovative findings, our data provide novel hypotheses related to the role of L1 in SLE pathogenesis. It focus attention on neutrophils as a potential source of L1-ORF1p, suggests that polymorphic variants of ORF1p (which could be mistakenly recognized as ?viral-derived? products) may trigger the humoral response against this protein, and opens the possibility that anti-ORF1p antibodies may be pathogenic in SLE. The major goal of this exploratory proposal is to gain further insights into the potential significance of these novel hypotheses and preliminary findings in the context of SLE pathogenesis. In Aim 1, we will use capture-based enrichment of L1-ORF1 combined with RNA- Seq to determine whether unique ORF1 transcriptional variants are differentially expressed in control and IFN- activated SLE neutrophils. In addition, we will define whether ORF1p variants are preferentially recognized by antibodies in SLE. In Aim 2, we will determine the prevalence and clinical associations of antibodies to ORF1p and their relationship to the IFN-signature in a prospective observational cohort of patients with SLE, for which extensive clinical and serologic data is available, as well as IFN-induced gene expression analysis. Together, these studies seek to enhance our understanding of self-immunogenic pathways underlying sterile inflammation in SLE. The final goal of this work is to gain new insights into disease mechanisms, thus laying the foundation to explore novel therapies.